A rolling string has one or more roll stands. The roll stands grip the workpiece strip with working rolls that are rotated oppositely to each other in the rolling direction. This rotation of the working rolls advances the workpiece. The height of the rolling nip determines the thickness to which the working rolls reduce the workpiece. A four-high roll frame (quarto) has two working rolls and two backing rolls. A six-high roll frame (sexto) has two working rolls, two outer backing rolls, and, between each outer backing roll and the respective working roll, an inner backing roll.
In order to roll very thin strip in a rolling string (hot or cold) small-diameter working rolls are used in the furthest downstream roll stand. They reduce the rolling force, the edge drop, and the temperature loss of the workpiece and also normally spare the roll stands of the rolling string because greater thickness reductions can be achieved.
Normally it is desirable to be able to hot roll not only very thin strip, but a thickness range from for example 0.8 mm to 12.7 mm. As a result of this requirement with respect to productivity, optimizing the roll string for thin strip is not possible for the following reasons:
Optimizing requires that for example the furthest two downstream roll stands with small-diameter working rolls be driven. The drive stubs and shafts of these small-diameter working rolls are often overloaded when rolling thick and/or hard strip. Switching out the small-diameter working rolls for large-diameter ones requires the drive shafts to be changed. The resultant down time of the rolling string is not acceptable.
German patent document 3,411,853 (US equivalent U.S. Pat. No. 4,674,313) describes a four- or six-high roll stand with driven backing rolls where the working rolls are driven frictionally by the backing rolls. In order to modify the operational characteristics of the rolling stand when it is driven at only a fraction of its capacity and such that the drive can be controlled accurately and loaded efficiently, it has been suggested that one of the drive motors be connected via a transmission with its backing roll and be uncoupled from this backing roll and shut down so that the load of the remaining drive motor is doubled and it can be controlled within the standard range more accurately.
The working rolls themselves have no drives but are in all case connected together by a synchronizing transmission so as to avoid any slip of the working rolls. Switching this type of rolling stand to large-diameter working rolls is neither foreseen nor possible, since it is impossible to drive the working rolls frictionally with a thicker strip exclusively from the backing rolls.